Many techniques have been devised for producing longitudinally welded tubes. However, problems have arisen in these past efforts which preclude the production of tubing having a strong and smooth welded seam.
In longitudinally welded tubes of the prior art, in particular those comprised of nonferrous metals, the welded seams evidence deficient grain structures as compared to the unwelded portions of such tubes. U.S. Pat. No. 3,650,456, discloses an apparatus for the continuous production of welded tubes from a continuous length of metal strip, whereby the strip is shaped into a tubular form so that the longitudinal edges of the strip are brought into abutting engagement, welded and then cold worked by the combination of a support mandrel positioned within the formed tube, an upper roller that engages an area of the outside surface of the welded seam for urging the seam toward the internal mandrel, and a lower support roller acting against the outside surface of the tube. The upper roller that engages the seam has a concave working surface that approximates that of the tube surface, and provides a high pressure condition in the area of contact with the seam to make the seam yield and assume a curvature approximating that of the remaining circumference of the tube.